Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes. Unfortunately, LEDs produce light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is typically utilized. For example, an LED-based light source that provides an emission that is perceived as matching a particular color can be constructed by combining light from red, blue, and green emitting LEDs. The ratios of the intensities of the various colors sets the color of the light as perceived by a human observer.
Unfortunately, the output of the individual LEDs vary with temperature, drive current, and aging. In addition, the characteristics of the LEDs vary from production lot to production lot in the manufacturing process and are different for different color LEDs. Hence, a light source that provides the desired color under one set of conditions will exhibit a color shift when the conditions change or the device ages. To avoid these shifts, some form of feedback system must be incorporated in the light source to vary the driving conditions of the individual LEDs such that the output spectrum remains at the design value in spite of the variability in the component LEDs used in the light source.
White light sources based on LEDs are in backlights for displays and projectors. If the size of the display is relatively small, a single set of LEDs can be used to illuminate the display. The feedback photodetectors in this case are located in a position that collects light from the entire display after the light from the individual LEDs is mixed.
As the size of the display increases, an array of LED light sources is needed to provide uniform illumination over the entire array. Such an array complicates the feedback system. If the photodetectors are positioned in the mixing cavity, light from the entire display is collected and analyzed. Hence, only the overall light intensity level of each color can be adjusted by the feedback system. Thus, if a particular LED is performing differently from the others that supply light in that color, the feedback system cannot adjust just that LED.